Thermoelectric oxides for waste heat recovery
Abstract
Among various solid-state energy conversion methods, thermoelectricity deals with
direct inter-conversion of thermal and electrical energy. The efficiency of a thermoelectric
heat engine is related to a material dependent figure of merit, Z, given by S2σ/κ, where S is
the thermopower or Seebeck coefficient, σ and κ are the electrical and thermal
conductivities (lattice and electronic), respectively. In conventional thermoelectric
materials such as semiconductors, the density of states, chemical potential and the
scattering mechanism governs electrical conductivity and thermopower. Due to this
coupling between thermopower, electrical conductivity and electronic component of
thermal conductivity, achieving high Z has been a challenging task. There are several
proposals and/or demonstrations to design high efficiency thermoelectric materials such as
“electron crystal, phonon glass” paradigm, [1] band engineering, [2] quantum confinement,
[3] electron filtering [4] etc. The decoupling of lattice component of thermal conductivity
from other parameters has been a very successful means to enhance the figure of merit, but
further decoupling of parameters like electrical conductivity and thermopower is necessary
to achieve high efficiency thermoelectric materials. From the technological perspective, it
is essential to design highly stable, cheap and abundant materials with good efficiency for
thermoelectric power generation. Complex oxides are an interesting class of materials,
which provides a chemically tunable platform to realize a wide range of physical
phenomena such as high temperature superconductivity (cuprates), ferroelectricity
(titanates, ferrites), magnetism (manganites, cobaltates) etc. Due to this versatility, they can
cater to both the scientific questions on thermoelectricity besides providing useful materials
for technological applications. The aim of our research during the fellowship was to
understand the nature of thermoelectricity in complex oxides and tailor these materials to
show enhanced thermoelectric properties using existing or new physical principles
discussed earlier.
The specific goals of my research was two fold:
(1) Understand the cross coupling between thermopower and electrical conductivity in
complex oxides and identify the possibilities of decoupling and simultaneously
increasing both properties with tunable material parameters.
(2) Explore the limits of phonon scattering at the interfaces of the oxide surfaces,
particularly in the form of superlattices and use it to enhance the thermoelectric
response in their doped analogues.